The current invention is directed to a method for reducing the propylene chlorohydrin byproduct from reactions involving hydroxypropylated hydrocolloids. More specifically the disclosed process is capable of reducing the amount of propylene chlorohydrin to less than 1 ppm in a hydroxypropylated/crosslinked hydrocolloids, with minimal crosslink stripping.
Hydroxypropylation is a reaction used to modify hydrocolloids and starch (whether or not he starch is functionally a hydrocolloid). The base polymer is modified using propylene oxide (PO). When a crosslinking agent, such as phosphorus oxychloride (POCl3), is added to the hydroxypropylated product can be referred to as hydroxypropylated/crosslinked. A product that is only subject to a crosslinking agent is referred to as crosslinked.
Hydroxypropylation and crosslinking are used to modify the functionality of the polymer. Considering starch specifically, hydroxypropylated starches are commonly used to provide viscosity and freeze-thaw stability in food products such as puddings, fruit pie fillings, sauces, salad dressings, and gravies. Hydroxypropylated/crosslinked starches further provide enhanced product texture, viscosity, and stability for processing and storage.
The process of hydroxypropylating a biopolymer, however, creates propylene chlorohydrin (“PCH”), an undesirable byproduct. By regulation of the Joint Expert Committee on Food Additives, PCH must be less than 1 ppm in modified food starches. The standard process for making hydroxypropylated/crosslinked starch yields PCH that exceeds these levels, requiring extra washing of the starch to remove the byproduct. But extensive washing increases starch losses and increases the amount of waste water to clean. So there is interest in other methods of reducing PCH.
One method, which is described in U.S. Pat. No. 4,431,800, takes advantage of the volatility of the propylene oxide, also called PO (a precursor to PCH) by aerating the starch slurry at high pH after completing the hydroxypropylation reaction. The '800 patent reports bench studies running for between 0.25 and 24 hours. At commercial scale, however, due to mixing and diffusion limitations, these times scale to become impracticable. Additionally, it is known that holding a hydroxypropylated/crosslinked starch at high pH tends to strip the crosslinking, making the method particularly unfit for reducing PCH during a process to make a hydroxypropylated/crosslinked starch.
So there is a need for a method of making a hydroxypropylated/crosslinked biopolymers that has less PCH, that uses less water and so produces less water to clean, and that does not strip the crosslinking from the starch.